The Agrobacterium rhizogenes rolB and rolC promoters are expressed in pericycle cells competent to serve as root initials in transgenic hybrid aspen

Abstract

Expression of the Agrobacterium rhizogenes rolB and rolC promoters was studied in transgenic hybrid aspen (Populus tremula L. ×P. tremuloides Michx.) lines containing a chimeric fusion of either the rolB or the rolC promoter and the reporter gene uidA. The resultant GUS activity was monitored by histochemical analysis in aerial tissues as well as in developing roots. Both the rolC and rolB promoters were shown to be expressed in the phloem and in the root tips, which is similar to the expression pattern previously described for annual plants. However, a strong expression of the rolB promoter in the rays of the phloem and the cambial zone of the stem, and of the rolC promoter in groups of pericycle cells prior to and during lateral root initiation was unique for hybrid aspen. In both stem and root tissues, the expression of the rolB and rolC promoters was localised primarily in a subset of cells competent to form adventitious or lateral roots, suggesting that these cells might serve as the target for A. rhizogencs infection. The biological significance of the cell‐specific rol gene expression in establishing the hairy root disease is discussed.